I am using the AD7793 to measuring T-type thermocouples, the input circuit is shown in the attachment. I had two small batches of PCBs, one worked but the other wouldn't output Vbias voltage. After examination I discovered that the manufacturer had fitted a uni-polar diode for D3 & D4 and then didn't check the orientation, this pulled the VBias pin to 0V. I assumed that removing the diodes would solve the issue but this seems to have damaged the VBias outputs and I had to have the AD7793s replaced. Does the VBias output have any protection against 0V?